The present invention relates generally to magnetic transducers for reading information signals recorded in a magnetic medium and, more particularly, to a magnetoresistive read sensor based on the giant magnetoresistance exhibited by multilayered ferromagnetic structures wherein the ferromagnetic layers are antiferromagnetically coupled.
It is well-known in the prior art to utilize a magnetic read transducer referred to as a magnetoresistive (MR) sensor or head for reading high density recorded data from magnetic media. An MR sensor detects magnetic field signals through the resistance changes of a read element fabricated of a magnetic material as a function of the strength and direction of imagnetic flux being sensed by the read element. These prior art MR sensors operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine (cos.sup.2) of the angle between the magnetization and the direction of sense current flow through the read element. A more detailed description of the AMR effect can be found in "Memory, Storage, and Related Applications", D. A. Thompson et al., IEEE Trans. Mag. MAG-11, p. 1039 (1975).
U.S. Pat. No. 4,896,235 entitled "Magnetic Transducer Head Utilizing Magnetoresistance Effect", granted to Takino et al on Jan. 23, 1990, discloses a multilayered magnetic sensor which utilizes the AMR and comprises first and second magnetic layers separated by a non-magnetic layer in which at least one of the magnetic layers is of a material exhibiting the AMR effect. The easy axis of magnetization in each of the magnetic layers is set perpendicular to the applied magnetic signal such that the MR sensor element sensor current provides a magnetic field in the magnetic layers parallel to the easy axis thus eliminating or minimizing Barkhausen noise in the sensor. "Thin Film MR Head for High Density Rigid Disk Drive" by H. Suyama et al, IEEE Trans. Mag., vol. 24, No. 6, 1988 (pages 2612-2614) discloses a multilayered MR sensor similar to that disclosed by Takino et al.
Magnetoresistive read sensors utilizing the AMR effect are generally superior to inductive sensors for a number of reasons, the most noteworthy being the larger signal and signal to noise ratio independent of relative motion between the sensor and magnetic media. However, at data recording densities greater than 5 gigabits/in.sup.2, it is expected that AMR sensors will provide insufficient sensitivity. The loss of signal strength at these densities is due largely to the thinning of the MR sensor stripe required at higher recording densities. Additionally, the AMR effect diminishes rapidly for sensor stripe thickness less than 10 nanometers (nm); for example, at a thickness of 3 nm, the deltaR/R is about 0.5 percent.
A second, different and more pronounced magnetoresistive effect has also been described in which the change in resistance of a layered magnetic sensor is attributed to the spin-dependent transmission of conduction electrons between ferromagnetic layers via a nonmagnetic layer separating the ferromagnetic layers and the accompanying spin-dependent scattering at the layer interfaces. This magnetoresistive effect is variously referred to as the "giant magnetoresistive" or "spin valve" effect. Such a magnetoresistive sensor fabricated of the appropriate materials provides improved sensitivity and greater change in resistance than observed in sensors utilizing the AMR effect. In this type of MR sensor, the in-plane resistance between a pair of ferromagnetic layers separated by a non-magnetic layer varies as the cosine (cos) of the angle between the magnetization in the two layers.
In a paper entitled "Enhanced Magnetoresistance in Layered Magnetic Structures with Antiferromagnetic Interlayer Exchange" The American Physical Society, Physical Review B, Vol. 39, No. 7, 1 Mar. 1989, G. Binasch et al describe a strong increase of the magnetoresistance effect as a result of antiferromagnetic exchange coupling between adjacent layers of iron separated by a thin nonmagnetic interlayer of chromium. U.S. Pat. No. 4,949,039 to Grunberg describes a layered magnetic structure which yields enhanced MR effects caused by antiparallel alignment of the magnetizations in the magnetic layers. As possible materials for use in the layered structure, Grunberg lists ferromagnetic transition metals and alloys, but does not indicate preferred materials from the list for superior MR signal amplitude.
U.S. Pat. No. 5,206,590, assigned to the instant assignee, discloses an MR sensor in which the resistance between two adjacent ferromagnetic layers separated by a thin layer of nonmagnetic material is observed to vary as the cosine of the angle between the magnetizations of the two layers and which is independent of the direction of current flow through the sensor. This mechanism produces a magnetoresistance that is based on the spin valve effect and, for selected combinations of materials, is greater in magnitude than the AMR.
The spin valve structures described in the above-cited U.S. patent require that the direction of magnetization in one of the two ferromagnetic layers be fixed or "pinned" in a selected orientation such that under non-signal conditions the direction of magnetization in the other ferromagnetic layer is oriented perpendicular to the pinned layer magnetization. Additionally, in both the AMR and spin valve structures, in order to minimize Barkhausen noise, it is necessary to,provide a longitudinal bias field to maintain at least the sensing portion of the read element in a single magnetic domain state. Thus, a means for both fixing the direction of the magnetization and providing a longitudinal bias field is required. For example, as described in the above-cited patent application and patents, an additional layer of antiferromagnetic material can be formed in contact with the ferromagnetic layer to provide an exchange-coupled bias field. Alternatively, an adjacent magnetically hard layer can be utilized to provide hard bias for the ferromagnetic layer.